In a fuel cell of a redox flow battery, reagents are passed to opposite sides of an ionically selective and conductive membrane separator separating two reagent compartments, electrodes being provided in the respective compartments. In a redox fuel cell, the reagents are an anolyte and a catholyte. Electricity flows between the reagents via the electrodes, as a result of ionic flow between the reagents through the separator. During discharge, these flows are in one direction and during charging they are reversed. Further, during discharge, the redox ions (not the ions passing through the separator) in the anolyte are oxidised and other redox ions in the catholyte are reduced. During conventional charging, the anolyte ions are reduced and the catholyte ions are oxidised. A small leakage of redox ions may occur through the separator. In the preferred redox flow batteries the redox ions are of the same metal, vanadium, and as such their transfer between electrolytes causes no contamination.
Such a redox flow battery requires reservoirs of the electrolytes and means for supplying them to and from the reagent compartments, e.g. pipes and pumps.
As used in this specification, the term “redox flow battery” is used to mean at least one redox fuel cell per se, together with a pair of electrolyte reservoirs, i.e. one for anolyte and the other for catholyte, and electrolyte supply means, i.e. means for circulating anolyte from its reservoir, to its compartment in the cell and back to the reservoir and like circulating means for catholyte. Usually, the battery will have several cells each supplied with anolyte and catholyte from a single pair of reservoirs.
Conventional charging, by application of a voltage to cause current flow in the reverse direction to that occurring during discharge, is slow, being of the same order of time as that for discharge or taking even longer. This is generally inconvenient for an automobile. Further, facilities for electrical recharging are inconvenient in a car park for instance and the time required for recharging may be insufficient when the automobile has been used to close to the extent of its range and the return journey is required to be made shortly after the outwards journey. It is possible to recharge more quickly, but this involves electric currents and voltages which are undesirably high for a car. Further, in electrically powered commercial vehicles, the time required for conventional recharging drastically reduces the available working time and places severe limitations on the use and applications of electric vehicles operating in a high pressure working environment.